The projects described in this application plan to examine various aspects of self:nonself discrimination with particular emphasis on mechanisms which may be involved in tumor immunity. Three different approaches to study self:nonself recognition are proposed, based on the expertise of three groups of investigators. Through the use of cell adhesion techniques, Project I will probe the nature of malignancy by comparing adhesion characteristics of malignant cells with those of normal cells. Project III employs immunochemical techniques to examine the structure of macrophage components and to relate understanding of the molecular nature of surface molecules to macrophage function in immune responses. A major aspect of self:nonself recognition manifests itself as the ability of the developing organism to recognize and become tolerant to self antigens, but not to nonself antigens, a process which will be studied in depth by Project IV.